1. Field of the Invention
This invention relates to a method of manufacturing an image-forming apparatus such as a display apparatus in which images are formed by irradiation of electron beams and it also relates to an image-forming apparatus manufactured by using said method.
2. Related Background Art
Known electron-emitting elements are currently classified into two categories. Those that are used as thermoelectron sources and those used as cold cathode electron sources. Of these, cold cathode electron sources are normally grouped as one of several types including the field effect emission type (hereinafter referred to as FE type), the metal/insulation layer/metal type (hereinafter referred to as MIM type) and the surface conduction type.
Some FE type devices are proposed in W. P. Dyke & W. W. Dolan, "Fieldemission", Advance in Electron Physics, 8,89 (1956) and C. A. Spindt, "Physical properties of thin-film field emission cathodes with Molybdenum cones", J. Appl. Phys, 47,5248 (1976).
On the other hand, C. A. Mead, "The tunnel-emission amplifier" J. Appl. Phys, 32,646 (1961) describes MIM type devices.
Finally, M. I. Elinson, Radio Eng. Electron Phys., 10 (1965) discloses certain surface conduction electron-emitting elements.
A surface conduction electron-emitting element is a device that utilizes the phenomenon of electron emission that takes place when an electric current is made to flow through a small thin film formed on a substrate in parallel with the surface of the film. Several different surface conduction electron-emitting elements have been reported, including the one comprising an SnO.sub.2 thin film as disclosed by Elinson cited above as well as those comprising an Au thin film [G. Dittmer: "thin Solid Films", 9,317 (1972)], an In.sub.2 0.sub.3 /SnO.sub.2 thin film [M. Hartwell and C. G. Fonstad: "IEEE Trans. ED Conf"., 519 (1975)] or a carbon film [H. Araki et al.: "Vacuum, Vol. 26, No. 1, p. 22 (1983)].
FIG. 7 of the accompanying drawings schematically illustrates a device proposed by Hartwell as cited above. Referring to FIG. 7, an electron-emitting region generating thin film 232 is formed of a metal oxide to show an H-shaped pattern on an insulator substrate 231 by sputtering and an electron-emitting region 233 is produced out of the thin film by means of an electrification treatment which is also called a forming operation. Reference numeral 234 denotes a part of the thin film including an electron-emitting region.
A surface conduction electron-emitting element having the above described configuration is normally subjected to an electrification treatment, which is also called forming, in order to produce an electron-emitting region 233 out of the electron-emitting region generating thin film 232. More specifically, forming is an operation of processing a surface conduction electron-emitting element where a voltage is applied to opposite ends of the electron-emitting region generating thin film 232 in order to produce an electrically highly resistive electron-emitting region 233 out of it by locally destroying or deforming it. Once subjected to a forming operation, the surface conduction electron-emitting element emits electrons from the electron-emitting region 233 when a voltage is applied to the thin film 234 including the electron-emitting region 233 to cause an electric current to run through the element.
However, conventional surface conduction electron-emitting elements are accompanied by certain known problems when they are used for practical applications. The applicant of the present patent application has been engaged in a series of research and development efforts in an attempt to solve the problems, which will be described hereinafter.
For example, the applicant of the present patent application has proposed an improved surface conduction electron-emitting element as shown in FIG. 8 (disclosed in Japanese Patent Application Laid-open No. 2-56822) comprising a film of fine particles 244 formed on a substrate 241 between a pair of electrodes (242, 243) as an electron-emitting region generating thin film, which is subjected to an electrification treatment to produce an electron-emitting region 245 out of it.
A large number of surface-conduction electron-emitting devices can be arranged in an array to form a matrix of devices that operates as an electron source, where the devices of each row are wired and regularly arranged to produce columns. (See, for example, Japanese Patent Application Laid-open No. 64-31332 of the applicant of the present patent application.)
Meanwhile, in recent years, flat panel display devices utilizing liquid crystal have been widely used in place of CRTs for image forming apparatuses, although such display devices are disadvantageous in that they are not of emissive type and hence require a light source such as a back light to be installed for operation. Therefore, there has been a strong demand for emissive type display devices.
Emissive type high quality display apparatus having a large display screen have been proposed to meet the demand. Such an apparatuses typically comprises an electron source having a large number of surface conduction electron-emitting elements arranged in array and a phosphor layer designed to emit visible light upon receiving electrons emitted from the electron source. (See inter alia U.S. Pat. No. 5,066,883 of the applicant of the present patent application.)
Now, the basic configuration of an image forming apparatus comprising electron-emitting elements will be summarily described below by referring to FIGS. 4 and 5.
As shown in FIGS. 4 and 5, an image forming apparatus comprises a number of electron-emitting elements 81 arranged on a substrate 85, a face plate 83 typically made of transparent glass, a phosphor layer 84 formed by applying phosphor to the inner surface of the face plate 83, a metal back layer 88, spacers 82 for separating the substrate 85 and the face plate 83 by a given distance, pieces of frit glass 86 for bonding the spacers 82, the face plate 83 and the substrate 85 together to form an envelope of the apparatus and hermetically sealing the envelope and an exhaust pipe 87 for evacuating the envelope. An envelope may alternatively be constituted of an integrally formed face plate 83 and spacer 82 or an integrally formed substrate 85 and spacer 82. The envelope is normally evacuated to a pressure of not higher than 10.sup.-6 torr.
With an image-forming apparatus having a configuration as described above, electron beams are emitted from the electron-emitting elements 81 in accordance with input signals as a high voltage of the order of several kilovolts is applied to the metal back layer 88 so that the emitted electron beams are accelerated before they hit the phosphor layer 84 to produce luminous images on the phosphor layer 84 as a function of input signals.
While an image-forming apparatus comprising an electron source formed by arranging a large number of electron-emitting elements in array is expected as a matter of course to have a large high quality image display screen, it has been proved that such a display screen is not easily obtainable particularly because of manufacture-related problems including the following.
First, during the operation of melting frit glass and bonding the face plate 83, the spacers 82 and the substrate 85 together with molten frit glass to produce an envelope, the entire image-forming apparatus needs to be heated to a temperature as high as 430.degree. C. for approximately sixty minutes to subsequently form an oxide film on the element electrodes of each of the electron-emitting elements and the wiring electrodes for wiring the electron-emitting elements, which by turn can significantly increase the electric resistance of the elements and the wires connecting them. The increase in the electric resistance of the electron-emitting elements and the wires results in a rise of electric energy consumption.
Secondly, it is very difficult to ensure an even distribution of temperature for the apparatus during the above described melting and bonding operation and consequently, the produced oxide film have a thickness and an electric resistances that may vary depending on the location where it is formed. As a result, the electron-emitting elements may emit electrons at different rates to produce improperly illuminated images on the display screen.
Finally, the metal of the element electrodes of the surface conduction electron-emitting elements is apt to be oxidized during the operation particularly at the interfaces of the thin film including an electron-emitting region and the element electrodes of each element to increase the electric resistance of the element so that, at worst, no electricity may be allowed to flow therethrough, making the element totally inoperative. If the operation of forming is carried out for the surface conduction electron-emitting elements after the above described melting and bonding operation, the operation of forming will consume electric energy at an enhanced rate because of the increased electric resistance of the elements due to the melting and bonding operation.